1. Field of the Invention
This invention relates generally to a process for removing boron from sulfuric acid. More particularly, this invention relates to a process for removal of boron from sulfuric acid by the addition of a fluoride in combination with water to increase the volatility of boron as boron trifluoride. Reduction of the boron content by vaporization of boron trifluoride improves the quality of the sulfuric acid for use in certain areas as for example the fertilizer industry.
2. Prior Art
Boron trifluoride is readily produced by the reaction of boric acid dissolved in concentrated sulfuric acid with fluorosulfuric acid which is also known as fluosulfonic acid. The reaction is frequently represented by the following equation: EQU B(OH).sub.3 +3FSO.sub.3 H.fwdarw.BF.sub.3 +3H.sub.2 SO.sub.4 I.
Equation I is convenient for describing the overall process and for determining material balances but is inaccurate in describing the actual reaction which takes place. Boric acid, B(OH).sub.3, does not exist as such when it is dissolved in concentrated sulfuric acid. Instead it is converted to tetra(hydrogensulfato)boric acid, HB(HSO.sub.4).sub.4, by the reaction shown as Equation II: EQU B(OH).sub.3 +4H.sub.2 SO.sub.4 .fwdarw.HB(HSO.sub.4).sub.4 +3H.sub.2 O II.
The mixture of tetra(hydrogensulfato)boric acid and water, both of which remain dissolved in the concentrated sulfuric acid solvent, are then reacted with fluorosulfuric acid, according to Equation III: EQU HB(HSO.sub.4).sub.4 +3H.sub.2 O+3FSO.sub.3 H.fwdarw.BF.sub.3 +7H.sub.2 SO.sub.4 III.
It will be noted that Equation I is the net result of adding Equation II and Equation III together. The process is described in detail in U.S. Pat. No. 2,416,133. While the process is comparatively simple to operate and anhydrous boron trifluoride is easily recovered, it suffers from some disadvantages. The chief disadvantage of the process is the generation of about 4.3 pounds of by-product sulfuric acid, which is contaminated with boron, for every pound of boron trifluoride produced. This contamination lessens the usefulness of the sulfuric acid and makes marketing more difficult. For example, boron is phytotoxic to many plants and, therefore, its presence in fertilizers above certain concentrations is undesirable. This limits the use of the sulfuric acid in the production of phosphate fertilizers which otherwise would be a natural outlet for the sulfuric acid. Also, the loss of boron values with the by-product acid imposes an economic burden on the process. Consequently, it is highly desirable to remove boron from the by-product sulfuric acid as recoverable boron trifluoride.
Several processes have been proposed for purification of boron-containing sulfuric acid. Among the suggestions for purifying contaminated sulfuric acid is a process described in U.K. Pat. No. 1,216,065. In this process nitrogen is passed through the contaminated sulfuric acid or the acid is contacted in a washing tower with air in countercurrent flow at elevated temperatures with absorption of the expelled boron trifluoride in water or formation of hydroxyfluoboric acid. This process suffers from a number of inherent disadvantages. For example, such a process is capable of recovering only that portion of the boron present as boron trifluoride. Furthermore, the process recovers the boron trifluoride in the form of hydroxyfluoboric acid which is unsuitable for conversion back to boron trifluoride. The resultant aqueous solution of hydroxyfluoboric acid represents a disposal problem rather than a valuable commodity. Moreover, substantially complete removal of boron trifluoride from inert gases (such as air) is not easily achieved with water as the absorbing liquid. The gas outflow has a fog of boron trifluoride in the form of a hydrate or other unknown species, representing a serious pollution problem.
Processes have been proposed to obviate the disadvantages of processes of the type described in UK Pat. No. 1,216,065. Examples of these processes are the processes described in U.S. Pat. No. 4,265,871 and German Pat. No. 3,017,156. These patents describe a process for purification of boron-containing sulfuric acid which consists of adding a quantity of fluorosulfuric acid in excess of the stoichiometric amount required to convert all of the tetra (hydrogensulfato) boric acid into boron trifluoride. The volatile boron trifluoride is then stripped from the acid at an elevated temperature using an inert carrier gas. A further embodiment of these patents is the recovery of the stripped boron trifluoride by absorbing it in a solvent consisting of boric acid dissolved in sulfuric acid. The absorbed boron trifluoride may then be recycled by feeding the solution to the boron trifluoride generator. The stripping process is based on the fundamental approach of shifting Reaction III towards the production of boron trifluoride at the expense of boric acid by increasing the concentration of fluorosulfuric acid relative to tetra(hydrogensulfato)boric acid. The approach is successful but has some disadvantages. For example, large amounts of fluorosulfuric acid are initially consumed in order to raise the vapor pressure of boron trifluoride sufficiently and, if the excess fluorosulfuric acid is to be recovered, higher temperatures, higher flow rates of stripping gas and/or longer stripping cycles are required.